1. Field of the Invention
The subject matter disclosed generally relates to the field of commutation control for electric motors.
2. Background Information
Equipment used to manufacture parts may contain mobile stages or platforms. For example, semiconductor fabrication equipment may have a stage that moves a wafer relative to a laser. The mobile stages may be driven by stepper motors that incrementally move a table of the stage. The stepper motors are typically controlled by a computer that moves the stage in accordance with a software or firmware routine.
FIG. 1 shows a control system 1 that is used to move the stage. The system includes a motor driver 2 that is connected to a motorized positioner 3. The motorized positioner 3 may include a stepper motor attached to a table or platform. The table or motor may have a position sensor 4 that provides feedback information regarding the actual position of the stage.
The system receives a desired trajectory value from a trajectory generator 5. The feedback signal from the sensor 4 is summed with the desired value by summing junction 6 to produce an error value. The error value is processed by a position/velocity loop controller 7. The controller 7 generates a commutation signal that is provided to the motor driver 2 to move the positioner 3. The process of generating an error signal and processing the error to a commutation signal occurs once every xe2x80x9ccontrol cyclexe2x80x9d. The frequency of the control cycle is limited by the speed of the loop of controller 7.
FIG. 2 shows a conventional sinusoidal commutation at relatively low motor speeds. The controller provides a series of step functions generated at the end of each control cycle. At lower speeds the process works relatively smoothly.
FIG. 3 shows a conventional sinusoidal commutation at relatively fast motor speeds. The motor speed is such that the desired commutation rate is close to the control cycle of the control system. Here the frequency of generated commutation signals approximates the required commutation frequency required to run the motor at the desired speed. The overlap in frequencies can create a frequency beat that approaches the resonant frequency of the motor. This can increase the acoustic noise and can create a roughness in the operation of the motor.
One solution would be to increase the speed of the control cycle to far exceed the desired commutation speed. This would require a faster controller, which increases the cost of the system.
A motor control system that can control a motor. The motor is driven by a commutation signal that is a function of a desired trajectory value. The control system generates an error signal as a function of the desired trajectory value and a feedback signal. An error signal is created within a single control cycle. The system contains a controller that generates a plurality of commutation signals within each control cycle.